1. Field of the Invention
This invention relates to a process for the separation and recovery of substantially pure aluminum chloride from mixtures containing ferric chloride and other chlorinated impurities.
2. Description of the Prior Art
Aluminum chloride is an important chemical which has many industrial applications. For example, it is used as a catalyst in organic chemical syntheses and as a nucleating agent for titanium dioxide pigments. In addition, recent technological developments indicate the need for large tonages of aluminum chloride for the production of aluminum metal.
Current domestic production of aluminum chloride is carried out almost exclusively through direct chlorination of metallic aluminum of sufficient purity that purification of the aluminum chloride is unnecessary. However, methods for production of aluminum chloride which employ aluminum metal obviously would not be of use in processes for the production of aluminum. A more economical method of producing aluminum chloride from chlorination of an aluminous material is required.
Various methods have been suggested for the production of substantially pure aluminum chloride. In U.S. Pat. No. 3,786,135 there is disclosed a process for recovering aluminum chloride from the chlorination products of sodium contaminated alumina. That method is effected by cooling the gaseous chlorination products to a temperature below the chlorination reaction temperature but above the ambient condition condensation temperature of aluminum chloride. A substantial portion of the condensable constituents other than aluminum chloride are thereby separated from the gaseous products. The gaseous products are separated from the condensed material and then are cooled to a lower temperature which still is above the ambient condition condensation temperature of the aluminum chloride to condense additional material. The condensed material is again separated and the remaining essentially contaminant-free gaseous products are introduced into a fluidized bed of aluminum chloride. The gaseous products are cooled to a temperature substantially below the condensation temperature of aluminum chloride to cause direct sublimation of the aluminum chloride.
The disadvantage of this process is that the sodium contaminated alumina must be substantially free of iron, titanium and silicon impurities to achieve production of high purity aluminum chloride. Otherwise, the iron is present as a ferric chloride-aluminum chloride dimer in the gaseous products along with titanium chloride and silicon chloride which condenses with the aluminum chloride.
In U.S. Pat. No. 3,938,969 a method to overcome the disadvantages of condensation processes is presented. In accordance with that process, the aluminum chloride vapor selectively is dissolved from the chlorination product gases leaving the chlorinator. This is accomplished by a scrubbing solvent at atmospheric pressure. The aluminum chloride then is separated from the solvent by distillation at a pressure sufficient to condense the aluminum chloride as a liquid.
U.S. Pat. No. 4,070,448 discloses an improvement in the process of U.S. Pat. No. 3,938,969 to control the particle size of the sublimed aluminum chloride and minimize chloride losses. The chlorination products including aluminum chloride vapors are introduced into a first fluidized bed at an entrance velocity of 18 to 90 meters per second. The fluidized bed comprises aluminum chloride which has a particle size of from 1-500 microns and is maintained at a temperature of about 80.degree. C. to 110.degree. C. The uncondensed gas is removed from the first fluidized bed and introduced into a second fluidized bed at a velocity of 18-90 meters per second. The second bed comprises aluminum chloride similar to the first bed. The temperature is maintained in a range of from about 20.degree. C. to 50.degree. C. to condense the remainder of the chloride values from the gas.
U.S. Pat. No. 4,083,923 discloses a process for producing metallurgically pure aluminum chloride. An iron-containing aluminous ore is chlorinated at temperatures up to about 1000.degree. C. to produce a mixed chloride gas stream. The gas stream is contacted with a liquid metal chloride solvent in an absorption column to preferentially absorb aluminum and iron chlorides. The aluminum chloride and iron chloride condense in the column and are dissolved by the solvent. The solvent mixture is withdrawn from the absorption column and introduced into a distillation vessel maintained at an elevated pressure to distill the aluminum chloride and iron chloride. The aluminum chloride and iron chloride vapors then are introduced into a rectification column operated at 3 to 4 atmospheres. The aluminum chloride is partially purified by the rectification. The aluminum chloride product stream is withdrawn from the rectification column and bubbled through a bath of molten aluminum metal to remove the remaining impurities.
Other processes for the purification of aluminum chloride or other metal chlorides are disclosed in, for example: U.S. Pat. No. 2,245,358; U.S. Pat. No. 2,311,466; U.S. Pat. No. 2,533,021; U.S. Pat. No. 2,675,890; U.S. Pat. No. 2,836,547; U.S. Pat. No. 2,940,827; U.S. Pat. No. 2,977,292; U.S. Pat. No. 3,066,010; U.S. Pat. No. 3,114,611; U.S. Pat. No. 3,218,122; U.S. Pat. No. 3,388,993; U.S. Pat. No. 3,793,003; U.S. Pat. No. 3,906,077; U.S. Pat. No. 4,035,169; U.S. Pat. No. 4,083,926; U.S. Pat. No. 4,066,424 and U.S. Pat. No. 4,159,310.
Numerous of the above processes depend upon fractional condensation or distillation to purify and separate the aluminum chloride from admixture with ferric chloride. However, aluminum chloride and ferric chloride exhibit mutual solid state solubility and in the vapor state mixed dimer molecules are present. Thus, many of the processes fail to produce an aluminum chloride product of sufficient purity for use in electrolytic processes for the production of aluminum metal.